This PR fixes a bug where a user might choose `classic-dark` as a theme
in `studio` but then `docs` and `marketing` apps will look weird.
To test:
- Change the localStorage value of `theme` to `classic-dark`
- Open `www` and `docs` apps, they should look ok
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## Summary by CodeRabbit
* **New Features**
* Added a new "classic-dark" theme option for enhanced visual
customization.
* **Improvements**
* Unified and simplified theme handling across apps for more consistent
behavior.
* Improved system-theme detection and smoother transitions when
switching themes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Ivan Vasilov